I'll have to read Steven's book. I'm quite surprised at his recommendation. Everything else I thought I knew about wedges said 'yes' to side to side and fore and aft. Basically, you are making a far shorter and stiffer column out of the mast when you restrain it. Euler's Law; 'fixed ended' versus 'pin ended' column.
Our North East 38 flexes enough to pop the saloon table support (looks alot like your centerboard cable cover) out of its socket (over 1/2") but she does not spit her wedges. Our Morgan 27 flexed enough, before I tied the deck-mounted chainplates down to the hull, to lift the deck 3/4" or so and spit the wedges. Dad had made special wood wedges with ell-ed tops (so they would not drop out), I replaced them in neoprene. Either way, they had to be restrained in place with an oversized hoseclamp, ugly but effective. This flexure is due to the compression load in the mast, when the rig is loaded, pushing the keel down with respect to the sides of the hull; and the deck either buckles inward or up. I can hear all sorts of movement in the NE 38's cabinetwork when this happens, and watch the table support post move in its socket; I could watch the Morgan's deck lift off the bulkheads. Additionally, the Morgan's forward keel bolt would leak when her rig was loaded.
On your Challengers, with the multipart deck structure and rather light side decks, what's moving, and how much more is it moving than when they were new, stiff boats? Certainly the NE 38 is, and the M 27 was, becoming more flexible with age.
And, what should we be doing about it?